It'll sink. We had a neighbor who let his cover fill up with rainwater. It took a few days but it sank in about 6' of water in his boat slip.

It really doesn't need to sink to the bottom to be junk though. If it sinks to just the surface, you will have so many problems with the boat you won't want it any more, engine, starter, alternator, batteries, fuel system, steering, all the electronics, ignition, etc. Everything will need to be gone thru.

I have no idea if there is any more or less flotation in there. But at that age, it may be water logged or not have as much as a newer boat.